[OT-Semi] On Linux, Permissions and Madness

Bob Sneidar bobs at twft.com
Wed Dec 1 16:47:05 EST 2010


Good work Andre. I don't know if Linux has something similar to Propagate Permissions like the Mac OS does. There is also a utility called BatCHMod for the Mac that provides a graphic interface for the chmod command. There may be something similar for Linux. 

Also, the Mac has a Check Permissions function in the Disk Utility, I wonder if there is something similar for Linux? 

Bob


On Dec 1, 2010, at 1:00 PM, Andre Garzia wrote:

> Folks,
> 
> Here goes a cautionary tale about madness, linux, php and huge web
> development support task at hand. As many here know, I've got a part time
> job writing PHP code for a company here in Brazil. The product I work is a
> web application for email marketing (a.k.a. spam) and is used by many of the
> biggest companies in Brazil to send their newsletters, promotions and
> general stuff you'd rather not be receiving (by the way, our OptOut works).
> 
> Our main web interface machine was an old slackware (slax64) box but we were
> doing all our development under CentOS. There is only my boss and me here on
> this mailsending coding business. We decided that this made no sense at all
> and decided to upgrade the main interface machine to CentOS. The process was
> done during a weekend and it was working fine until the middle of the day of
> wednesday when lots of features started breaking.
> 
> After some investigation, we noticed that our PHP code was receiving a
> "permission denied" error whenever it tried to execute some external
> command, like doing shell() with LiveCode. We used a lot of shell commands
> to call Tidy utility to validade HTML, to call SpamAssassin to evaluate what
> would be sent for spam potential and more. Our first thought was to check if
> we had php safe_mode on, which is like LiveCode secure mode. It was off.
> Then we checked suEXEC but it was ok and we started checking everything
> possible under the sun. I am familiar with linux and know my way around and
> I could see nothing wrong. All the binaries had the correct permission set
> and were owned by the correct groups and users. We lost two days searching
> for it and this includes going to sleep at 2:00 AM trying to find why
> "permission denied" was there. The clients were going crazy too.
> 
> Today I came early to the office and started working on this trouble but
> could not find why the problem was happening. We started trying heroic
> solutions. Heroic solutions are the ones you are sure will never work but
> you try them our of desperation with faith in the remote possibility they
> might solve something. No heroes here. We even upgraded the development box
> to run the exact same version of CentOS to try to replicate the problem but
> we could not.
> 
> I've launched Apache server with strace enabled to track all the system
> calls being done. I did this on a production server with massive access. I
> went crazy reading the logs and seeing those silly:
> 
>   stat("/usr/sbin/sh", 0x7fff52c83180) = -1 EACCES (Permission denied)
> 
> I've spent days and hours on this one. Then at 6:30 PM, out of desperation I
> decided to re-check all the permissions while trying to execute
> /usr/bin/tidy. Tidy was 755 so it was correct, the bin folder was correct as
> well, so I went on to check "/" which would be madness if it had the wrong
> perms, 755, right, so I went to check the "usr" folder...
> 
> OH HOLLY HACK!!!!!
> 
> The "usr" folder has perms 644, which means that the whole system was
> somewhat compromissed because no one would be able to execute anything. But
> since the permission is a 6xx, it means that the root user could execute
> stuff, that is why the system was still answering to me thru ssh and things
> appeared normal, it only failed for the other users, since there were no
> other user but root and apache and nobody, we never thought of that.
> Somehow, something, something EVIL changed the permissions there and it took
> us three days to figure out that up in the chain a folder had no execute
> permission.
> 
> So the next time, something does not execute on your linux box, check all
> the perms, from the root / folder down to the binary, somewhere in the
> middle might be a 644 and you will not loose time like I did.
> 
> This was really hard to find....
> 
> andre
> PS: I *NEED* a vacation...
> 
> 
> -- 
> http://www.andregarzia.com All We Do Is Code.
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